Synthesis of functionalized 2,2′- and 2,3′-bipyrroles via 3-imino-3H-pyrrolizine-2-carbonitriles

Article abstract of DOI:10.1016/j.tetlet.2016.07.006

A novel strategy for the synthesis of 4-amino-3-cyano-2,2′- and 5′-amino-4′-cyano-2,3′-bipyrroles via readily available 1-amino-3-imino-3H-pyrrolizine-2-carbonitriles has been developed. The rearrangement leading to 5′-amino-4′-cyano-2,3′-bipyrroles involves formation of the intermediate aziridine, which undergoes other side ring-opening.

Full text of DOI:10.1016/j.tetlet.2016.07.006

Tetrahedron Letters 57 (2016) 3652–3656
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Tetrahedron Letters
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Synthesis of functionalized 2,20- and 2,30-bipyrroles via 3-imino-3H-
pyrrolizine-2-carbonitriles
Olga V. Petrova, Elena F. Sagitova, Lyubov N. Sobenina, Igor A. Ushakov, Tat0yana N. Borodina,
⇑
Vladimir I. Smirnov, Boris A. Trofimov
A.E. Favorsky Irkutsk Institute of Chemistry, Siberian Branch, Russian Academy of Sciences, 1 Favorsky Str., 664033 Irkutsk, Russian Federation
a r t i c l e i n f o
a b s t r a c t
A novel strategy for the synthesis of 4-amino-3-cyano-2,20- and 50-amino-40-cyano-2,30-bipyrroles via
readily available 1-amino-3-imino-3H-pyrrolizine-2-carbonitriles has been developed. The rearrange-
ment leading to 50-amino-40-cyano-2,30-bipyrroles involves formation of the intermediate aziridine,
which undergoes other side ring-opening.
Article history:
Received 10 May 2016
Revised 28 June 2016
Accepted 1 July 2016
Available online 2 July 2016
Ó 2016 Elsevier Ltd. All rights reserved.
Keywords:
3-Imino-3H-pyrrolizine-2-carbonitriles
2,20-Bipyrroles
2,30-Bipyrroles
KOH/DMSO system
1-Chloroacetophenone
The bipyrrole scaffold occurs in many natural products,1,2 pig-
ments, porphyrin mimics,3 compounds used for anion recognition
and in the design of self-assembly systems.4 Bipyrroles have
sparked interest as building blocks for the synthesis of isomeric
and expanded forms of porphyrins such as rubyrin, rosarin,4
meso-substituted [34]octaphyrin(1.1.1.0.1.1.1.0) and cyclo[8]
pyrroles.5 They also attract attention as a synthetic intermediate
towards antitumor medicines (prodigiosins,6 sapphyrins7), radia-
tion sensitizers7,8 photodynamic cancer therapy and antimalarial
agents.9
yields, are known to date.9,10b–e,17–21 The most efficient of
these include phenyliodine(III) bis(trifluoroacetate)-induced
a
oxidative coupling of N-substituted pyrroles,10b–e the 1,3-dipolar
cycloaddition of N-protected 2-nitrovinylpyrroles with unsym-
metrical 1,3-oxazolium-5-olates,20 the Suzuki coupling of 4-
bromo-pyrrole-2-carboxaldehydes with N-Boc-2-pyrroleboronic
acid,9 and the reaction of (1-methoxy-3-(1-methyl-1H-pyrrol-2-
yl)prop-2-yn-1-yl)lithium with isothiocyanate.21 Most of these
were of narrow substrate scope.
It is important for this communication to note that only one
protocol for the synthesis of bipyrroles with an amino group has
been described,22 but none for a cyano substituent and, moreover,
for the combination of these two functionalities. The synthesis of
bipyrroles with amino substituents includes multi-step procedures
starting from 4-oxo-N-(PhF)prolinate. It is important to emphasize
that bipyrroles, both 2,20- and 2,30-bearing neighboring amino and
cyano functions, might be precursors of purine analogues, includ-
ing pyrrolotetrazines,23 pyrrolotriazines,24 pyrrolopyridines,25
pyrrolopyrimidines,26 and in this case, for their unknown families
with a second pyrrole ring.
The strong interest in this structural unit has resulted in the
development of a vast amount of synthetic research in this area.
Classic approaches to 2,20-bipyrrole syntheses include oxidative,10
Ullman-type homocouplings10a,11 or the condensation of
pyrrolinones with pyrroles.12 The first two methods are limited
to the preparation of symmetrical 2,20-bipyrroles, while the third
one allows a few unsymmetrical 2,20-bipyrroles to be synthesized.
The latter were sporadically obtained by Paal-Knorr condensa-
tion,13 Suzuki coupling,6a,d,9 aza-Nazarov14 and Trofimov reac-
tions,15 from 2-cyanopyrroles and substituted cyclopropanes,16 or
from pyrrolylbutenyl ketones.6b
Herein, we describe a convenient strategy for the synthesis of
In contrast to the numerous syntheses of 2,20-bipyrroles, fewer
methods for the preparation of 2,30-bipyrroles, mostly in low
2,20- and 2,30-bipyrroles with neighboring amino and cyano
groups. This is
a continuation of the previously developed
carbodithioation of substituted pyrroles with CS2 in a KOH/
DMSO system.27 The pyrrole-2-carbodithioates obtained were
then easily transformed into 1-amino-3-imino-3H-pyrrolizines
(Scheme 1).28,29
⇑
Corresponding author. Tel./fax: +7 3952 419346.
E-mail address: boris_trofimov@irioch.irk.ru (B.A. Trofimov).
http://dx.doi.org/10.1016/j.tetlet.2016.07.006
0040-4039/Ó 2016 Elsevier Ltd. All rights reserved.

O. V. Petrova et al. / Tetrahedron Letters 57 (2016) 3652–3656
3653
R2
R2
Table 2
Synthesis of 2,30-bipyrroles 4a–c from pyrrolizines 2a–c and 1-chloroacetophenone
i
ii
S
R1
R1
Ph
NH
NC
R2
R1
N
H
N
H
NH2
Ph
R2
R1
SEt
ClCH2COPh
R2
R2
N
N
KOH/DMSO
110-120 °C
6 h
N
H
iii
CN
NHR3
PhOC
R1
R1
SEt
HN
N
N
2a-c
4a-c
HN
CN
HN
CN
Entry
Pyrrolizine
2,3-Bipyrrole
NC
Yield (%)
1a-d (R3 = Me),
2a-c (R3 = Ph)
Et
NH2
Ph
Et
Pr
N
NHPh
Scheme 1. Synthesis of 1-amino-3-imino-3H-pyrrolizines 1a–d and 2a–c. Reagents
and conditions: i: (a) CS2, KOH/DMSO, (b) EtI; ii: (a) CH2(CN)2, KOH/DMSO, (b) EtI;
iii: R3NH2.
N
N
H
1
10
Pr
PhOC
CN
HN
Pr
2a
4a
The readily accessible pyrrolizines 1a–d, 2a–c served as starting
materials for the synthesis of functionalized 2,20- and 2,30-
bipyrroles.
When 1-methylamino-3-imino-3H-pyrrolizines 1a–d were
treated with 1-chloroacetophenone, 2,20-bipyrroles 3a–d were
formed in yields of 20–54% (Table 1).
The reaction was carried out by the portion-wise addition of
1-chloroacetophenone (1.75 equiv) to a stirred suspension of
pyrrolizine 1a–d (1 equiv) and KOH (1.5 equiv) in DMSO over 4 h,
followed by stirring for an additional 2 h. The excess of
1-chloroacetophenone and its gradual addition were found to be
NC
NH2
Ph
Pr
Bu
N
NHPh
N
N
H
2
3
7
Bu
PhOC
HN
CN
4b
2b
NC
NH2
NHPh
N
N
Ph
N
H
5
PhOC
CN
HN
2c
4c
Table 1
Synthesis of 2,20-bipyrroles 3a–d from pyrrolizines 1a–d and 1-chloroacetophenone
R2
R1
NC
R2
R1
O
NH2
NHMe
CN
Ph
N
Ph
R2
R1
ClCH2COPh
R2
NH
N
Ph
KOH/DMSO
110-120 oC
6 h
N
H
N
N
COPh
R1
N
CN
Me
3a-d
HN
H
CN
NC
HN
1a-d
2,20-Bipyrrole
Yield (%)
5a-c
6a-c
Entry
Pyrrolizine
Et
NC
Figure 1. 1-Anilino-2,2-dicyanoethenylpyrroles 5a–c and 3-imino-1-[(2-oxo-2-
phenylethyl)anilino]-3H-pyrrolizine-2-carbonitriles 6a–c.
Et
NHMe
NH2
N
Pr
N
1
2
54
N
Pr
H
CN
COPh
Me
HN
O
Ph
R2
R1
1a
1b
3a
NC
N
Ph
4a-c
Pr
KOH/DMSO
120 °C, 4 h
N
Pr
NHMe
CN
NH2
CN
N
HN
Bu
N
6a-c
N
48
Bu
H
COPh
Me
HN
Scheme 2. Synthesis of 2,30-bipyrroles 4a–c from ketones 6a–c.
3b
NC
NHMe
crucial to avoid the undesirable conversion of
the presence of base.30
Surprisingly, when the methylamino substituent in the pyrroli-
zine ring was replaced by a phenylamino group, 2,30-bipyrroles
4a–c, instead of the expected 2,20-bipyrroles, were isolated in
yields of 5–10% (Table 2).
a-halo ketones in
NH2
N
HN
3
4
N
H
40
20
N
CN
COPh
Me
NC
1c
3c
NHMe
In this case, along with 2,30-bipyrroles, 1-anilino-2,2-dicya-
noethenylpyrroles 5a–c and 3-imino-1-[(2-oxo-2-phenylethyl)ani-
lino]-3H-pyrrolizine-2-carbonitriles 6a–c were also formed in
yields of 35–41% and 14–15%, respectively (Fig. 1).
NH2
N
Ph
CN
N
H
N
Ph
COPh
HN
Me
The ketones 6a–c are likely to be intermediates in the synthesis
1d
3d
of 2,30-bipyrroles since their further heating (110–120 °C) in

3654
O. V. Petrova et al. / Tetrahedron Letters 57 (2016) 3652–3656
Ph
NH
KOH/DMSO for 4 h resulted in the formation of additional amounts
R2
R1
of the respective 2,30-bipyrroles (13–23%). Thus, the overall yield of
this two-step procedure ranges from 19% to 33% (Scheme 2).
It is worth noting that 1-anilino-2,2-dicyanoethenylpyrroles
5a–c, when reacted with 1-chloroacetophenone under the above
conditions, did not give either 2,20- or 2,30-bipyrroles suggesting
that they are not intermediates in the bipyrrole synthesis
(Scheme 3).
ClCH2COPh
3 or 4
N
H
KOH/DMSO
110-120°C
CN
NC
5a-c
Scheme 3. Reaction of 1-anilino-2,2-dicyanoethenylpyrroles 5a-c with 1-
chloroacetophenone.
As shown in the example of 1-anilino-3-imino-5,6,7,8-tetrahy-
dro-3H-pyrrolo[1,2-a]indole-2-carbonitrile (2c),
a temperature
increase (140 °C) did not improve the yield of 2,30-bipyrrole 4c.
In this case, enol 7, the tautomer of the expected ketone 6c, was
also isolated in 10% yield (Scheme 4).
NC
NH2
Ph
Ph
NH
ClCH2COPh
N
N
N
H
KOH/DMSO
140 °C
The same reaction using the Cs2CO3/DMSO system at 120 °C
gave a 13% (22% based on the conversion of 2c) yield of 2,30-bipyr-
role 4c and a 13% yield of enol 7, with the conversion of the starting
compound 2c being 59%.
PhOC
CN
HN
2c
4c (7%)
Ph
Ph
Furthermore, 2,30-bipyrroles 4a-c were obtained in 12–26%
yield via the reaction of pyrrolizines 2a–c with 1-chloroacetophe-
none in the K2CO3/acetone system (reflux, 3 h) to deliver ketones
6a–c (37–65% yield), which were converted to the target products
4a–c using the following conditions: KOH/DMSO, 120 °C, 4 h
(Table 3).
We also carried out the reaction of intermediate 6a with
t-BuOK/DMSO (120 °C, 2 h). Surprisingly, the expected
2,3-bipyrrole 4a was not formed. Instead, unidentified products
were formed in a gum-like reaction mixture (1H NMR data).
+
N
OH
N
H
CN
NC
7 (10%)
Scheme 4. Reaction of 1-anilino-3-imino-3H-pyrrolizine 2c with 1-chloroace-
tophenone in KOH/DMSO at 140 °C.
Table 3
Synthesis of 2,30-bipyrroles 4a–c via ketones 6a–c
O
Ph
R2
R1
Ph
R2
R1
NH
ClCH2COPh
N
Ph
KOH/DMSO
K2CO3/acetone
reflux, 3 h
N
N
120 °C, 4 h
CN
CN
HN
HN
2a-c
6a-c
NC
NH2
R2
N
Ph
N
H
R1
PhOC
4a-c
Entry
Ketone 6
Yield (%)
2,30-Bipyrrole
NC
Yield (%)
O
Ph
NH2
Ph
Et
Et
N
Ph
Ph
Ph
N
N
Pr
N
H
1
65
Pr
12
CN
PhOC
HN
4a
6a
6b
6c
O
NC
Ph
NH2
Ph
Pr
Pr
N
N
N
Bu
N
H
2
37
Bu
26
CN
PhOC
HN
4b
O
NC
Ph
N
NH2
N
N
Ph
N
H
3
54
14
CN
PhOC
HN
4c

O. V. Petrova et al. / Tetrahedron Letters 57 (2016) 3652–3656
3655
steric strain) to give the intermediate carbanionic aziridines C.
The latter undergo ring-opening from the other side to give ani-
line-anions D, which intramolecularly attack the cyano group to
afford pyrrolyliminopyrrolines E which are subsequently trans-
formed into 2,30-bipyrroles 4 (Scheme 6).
A driving force of the alternative recyclization in the case of
anilino derivatives 2a–c is likely to be the formation of N-phenyl
anionic moieties, more stable than the corresponding
N-methyl anionic intermediates, which could be formed from
N-methyl-amino pyrrolizines 1a–d.
In conclusion, the reaction of 1-amino-3-imino-3H-pyrrolizine-
2-carbonitriles with 1-chloroacetophenone in a KOH/DMSO or
Cs2CO3/DMSO system affords 4-amino-3-cyano-2,20-bipyrroles (in
the case of 1-methylamino-3-imino-3H-pyrrolizine-2-carboni-
triles) or 50-amino-40-cyano-2,30-bipyrroles (in the case of 1-ani-
lino-3-imino-3H-pyrrolizine-2-carbonitriles), bearing neighboring
amino and cyano functional groups. These previously inaccessible
functionalized bipyrroles may serve as precursors for purine ana-
logues with a pyrrole ring.
Figure 2. Single crystal X-ray structures of 2,20-bipyrrole 3a and 2,30-bipyrrole 4b.
O
Me
R2
R1
[OH-]
N
ClCH2COPh
KOH/DMSO
Ph
1a-d
N
CN
HN
9
Me
R2
COPh
Me
O
R2
Acknowledgments
N
N
3a-d
Ph
-
This work was supported by the President of the Russian
Federation [programs for the support of leading scientific schools
(grant NSh-7145.2016.3)] and using the equipment of Baikal
Analytical Center for collective use SB RAS.
N
R1
NH
N
H
R1
CN
NC
HN
A
10
Scheme 5. Proposed mechanism for 2,20-bipyrrole 3a–d formation.
Supplementary data
Ph
Ph
O
Supplementary data (experimental procedures and characteri-
zation data, copies of 1H, 13C NMR spectra and single crystal
X-ray data for 3a and 4b) associated with this article can be found,
in the online version, at http://dx.doi.org/10.1016/j.tetlet.2016.07.
006.
Ph
O
O
-
R2
R1
R2
R1
R2
R1
[OH-]
N
N
Ph
Ph
N
Ph
N
N
N
-
H
CN
CN
NC
CN
C
HN
6a-c
R2
R1
HN
B
References and notes
O
Ph
O
Ph
R2
R1
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1a–d are alkylated with 1-chloroacetophenone to give ketones 9.
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3656
O. V. Petrova et al. / Tetrahedron Letters 57 (2016) 3652–3656
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